/Engine/Source/Runtime/Core/Public/Containers/Map.h
Move all items from another map into our map (if any keys are in both, the value from the other map wins) and empty the other map.
template<typename OtherSetAllocator> void Append ( [TMultiMap](API\Runtime\Core\Containers\TMultiMap)< KeyType, ValueType, OtherSetAllocator, KeyFuncs > && OtherMultiMap )